NOD-like Receptor (NLR)

Nucleotide oligomerization domain (NOD)-like receptors (NLRs) are critical cytoplasmic pattern-recognition receptors (PRRs) that play an important role in the host innate immune response and immunity homeostasis. There are 23 NLR family members in humans and at least 34 NLR genes in mice. NLRs are expressed in many cell types including immune cells and epithelial cells, although certain NLR family members are expressed primarily in phagocytes including macrophages and neutrophils. The NLR family are most commonly classified according to their N-terminal domain, falling into one of four subfamilies; NLRA, NLRB, NLRC and NLRP.

The NLRs recognize various ligands from microbial pathogens (peptidoglycan, flagellin, viral RNA, fungal hyphae, etc.), host cells (ATPs, cholesterol crystals, uric acid, etc.), and environmental sources (alum, asbestos, silica, alloy particles, UV radiation, skin irritants, etc.). Most NLRs act as PRRs, recognizing the above ligands and activate inflammatory responses. However, some NLRs may not act as PRRs but instead respond to cytokines such as interferons. The activated NLRs show various functions that can be divided into four broad categories: inflammasome formation, signaling transduction, transcription activation, and autophagy.